With increased concern over environmental air quality, innovative solutions have been sought for adding filtration capacity to new and existing air circulation systems, such as heating, ventilation, and cooling systems (HVAC) for buildings and vehicles. For example, the HVAC systems in most vehicles do not include air filters. Minimal space is generally available for retrofitting a filter to the HVAC system. Moreover, it may be necessary to provide one filter for incoming air and a second filter for air recirculating within the passenger compartment. Even on new vehicles, space within the HVAC system is at a premium and it is difficult for some manufacturers to provide a location for an appropriate filter.
In addition to the difficulty of finding sufficient space for a filter, the failure mode of most filter media also raises concerns. Over time, environmental contaminants accumulate in filters, typically resulting in a reduced flow rate through the air circulation system. Failure to replace the filter media periodically can result in an increased static air pressure drop across the filter and reduced efficiency for the air circulation system. The reduced flow rate through a loaded filter can also create safety hazards, such as allowing insufficient air flow for operating the defrost system of an HVAC system.
One approach to retrofitting an air filter to an HVAC system of a vehicle is disclosed in U.S. Pat. No. 5,683,478 (Anonychuk). The air filter is sized and shaped to fit into a cavity located within a blower motor assembly. An outwardly extended lip is provided on the base of the air filter for rigid attachment to a rim located below the fan on the automobile. The fan in the blower motor assembly rotates around the stationary filter. Although the '478 patent recognizes the need to provide filtration efficiency without impeding air flow, air flow will inevitably be reduced as the filter becomes loaded with environmental contaminants. The failure mode of the filter element may be an unacceptable reduction in air flow through the blower motor assembly.
U.S. Pat. No. 5,265,348 (Fleishman et al.) discloses the use of a rotating foam material on a rotary fan to reduce noise.